1. Field of the Invention
The present invention relates to an internal combustion engine controller, and more particularly to an internal combustion engine controller, such as a power source device for an in-cylinder direct injection injector of a vehicular internal combustion engine, etc., suitable for use in driving a load using a high voltage obtained by boosting the voltage of a battery power source.
2. Background Art
As an internal combustion engine that runs on gasoline, diesel or the like, there is known an internal combustion engine of an in-cylinder direct injection scheme that injects fuel directly into the cylinder at the compression stroke by means of an injector (fuel injection valve) in order to improve fuel economy. In an internal combustion engine of the in-cylinder direct injection scheme, stratified combustion can be achieved where atomized fuel is present only around the spark plug to establish combustion, and the air therearound serves as thermal insulation for the cylinder walls and to press the piston down by means of thermal expansion from the energy at the time of combustion. Thus, as compared to an internal combustion engine that combines “port injection” and a “premixed combustion scheme” in which fuel is injected at the intake stroke, heat loss can be reduced, which results in better fuel economy.
In the case of an internal combustion engine of a scheme in which in-cylinder direct injection is performed, because fuel is injected by the injector at the compression stroke, highly pressurized fuel has to be used, and high energy is required for the valve opening operation of the injector due to the increase in fuel pressure. Further, in order to improve responsiveness to control and accommodate high speed revolution, this high energy has to be supplied to the injector within a short period of time.
A conventional example of a power circuit of the in-cylinder direct injection scheme (direct injection injector) will be described with reference to FIG. 15. The power circuit comprises a boost circuit 10 that boosts a voltage Vbat of a battery power source 1 to a higher voltage Vbst, and employs a scheme wherein the current flowing through an injector 3 is raised in a short period of time by the boost voltage Vbst that the boost circuit 10 generates (for example, see JP Patent Publication (Kokai) No. 2001-55948 A and JP Patent Publication (Kokai) No. 2009-22139 A).
Injector current Ainj shown in FIG. 16 is a current waveform of a typical direct injection injector. During peak current flowing time t1 at the initial stage of current flow, the injector current Ainj rises to a predefined peak current a1 over a short period of time using the boost voltage Vbst. This peak current a1 is greater by a factor of about 5 to 20 as compared to a case of a scheme in which a mixture gas of air and fuel is drawn into the cylinder at the intake stroke.
Upon expiration of the peak current flowing time t1, the power-source voltage of the injector 3 shifts from the boost voltage Vbst to the battery power-source voltage Vbat, and the injector current Ainj drops to a hold current a2 that is about ½ to ⅓ of the peak current a1. Thereafter, the injector current Ainj further drops to a hold current a3 that is about ⅔ to ½ of the hold current a2.
The injector 3 opens its valve by means of the peak current a1 and the hold current a2, and injects fuel into the cylinder. The time it takes to shift from the peak current a1 to the hold current a2 is determined by the magnetic circuit characteristics and fuel atomizing characteristics of the injector 3, the fuel pressure of the common rail that supplies fuel to the injector 3, and the power demanded of the internal combustion engine.
In order to achieve better fuel economy, the amount of fuel injected must be controlled with precision. To this end, it is necessary that the flowing current drop time t2 for the injector current Ainj be shortened and the injector current Ainj be dropped quickly so that the closing of the valve of the injector 3 would be carried out quickly. Further, it is also necessary to drop the current within a short period of time in the process of shifting from the peak current a1 to the hold current a2 and in the process of shifting from the hold current a2 to the even smaller hold current a3.
However, high energy is stored in the injector 3 due to the fact that the injector current Ainj is flowing. In order to drop the current, this energy needs to be eliminated from the injector 3. In order to realize this within the flowing current drop time t2 of a short duration, various schemes have conventionally been employed, examples of which include a scheme in which energy is converted to thermal energy using the Zener diode effect at a drive element of a drive circuit 4 of the injector 3, a scheme in which the injector current Ainj is recirculated to a boost capacitor 13 of the boost circuit 10 via a current recirculating diode 8, and so forth.
With the scheme in which energy is converted to thermal energy, it is possible to simplify the drive circuit 4. However, because the energy of the flowing current of the injector 3 is converted to thermal energy, it is unsuitable for drive circuits of large currents. In contrast, with the scheme in which current is recirculated to the boost capacitor 13, heat generation of the drive circuit 4 can be relatively suppressed even when a large current is flowing through the injector 3. Thus, this scheme is widely employed in internal combustion engines using a direct injection injector (sometimes referred to as “common rail engines”) that uses diesel which causes a large current to flow through the injector 3, and in internal combustion engines that use an in-cylinder direct injection injector that uses gasoline as fuel.
In the boost circuit 10 used for these purposes, the drive circuit 4 causes the injector current Ainj to flow through the injector 3 using the boost voltage Vbst. As a result, as shown in FIG. 16, when it is detected by a voltage detection portion 21 that the boost voltage Vbst has fallen to or below a boost start voltage b1, a boost control portion 29 starts a boost operation. Once the boost operation is started, the boost control portion 29 changes a boost control signal (c) for causing a current to flow through a boost switch element 16 from low to high. Thus, a current flows from the battery power source 1 to a boost coil 11, and energy is stored in the boost coil 11. The current that flows through the boost coil 11 is converted as a shunt resistor flowing current Ashu to a voltage by a shunt resistor element (boost switching current detection resistor element) 12, and is detected by a current detection circuit 26.
Once the shunt resistor flowing current Ashu reaches a predetermined switching stop current e1, the boost control portion 29 changes the boost control signal (c) that controls the opening/closing of the boost switch element 16 from high to low, and interrupts the shunt resistor flowing current Ashu. Thus, the current flowing through the boost coil 11 becomes unable to flow to the power source ground via the boost switch element 16, and the energy stored by the inductive component of the boost coil 11 generates a high voltage.
Further, when this voltage becomes greater than the combined voltage of the boost voltage Vbst stored in the boost capacitor 13 and the forward voltage of a charge diode 14, the energy stored in the boost coil 11 shifts as a charge current Acha to the boost capacitor 13 via the charge diode 14.
In so doing, the charge current Acha declines rapidly, along with the energy shift to the boost capacitor 13, from the switching stop current e1 that was flowing through the boost coil 11 immediately before the interruption by the boost switch element 16.
If it is detected by the voltage detection portion 21, which detects the boost voltage Vbst, that the boost voltage Vbst that rose through the operation above does not reach a predetermined boost stop voltage b2, the boost control portion 29, without detecting the charge current Acha, changes the boost control signal (c) from low to high in order to cause a current to flow through the boost switch element 16 in accordance with a predefined boost switching period (constant pulse width) t4. This operation is repeated in accordance with a boost voltage recovery time t3 that it takes for the boost voltage to reach the predetermined boost stop voltage b2.
Thus, as compared to a boost circuit that controls by means of a predefined time (a period in which the duration for changing the boost control signal (c) from low to high is made to be of a constant length) without detecting the shunt resistor flowing current Ashu, the boost circuit 10 that detects the shunt resistor flowing current Ashu and controls so that the predetermined switching stop current e1 is not exceeded is capable of keeping the shunt resistor flowing current Ashu low. As a result, it is possible to suppress heat generation from the boost switch element 16, the boost coil 11, and the charge diode 14 to a minimum.
Further, there has been proposed a power circuit that is configured in such a manner that the current upstream of the boost switch element 16 is monitored, and if this current drops to a predetermined current value, the boost control portion 29 switches the boost control signal (c) that controls the opening/closing of the boost switch element 16 from low to high (for example, see JP Patent Publication (Kokai) No. 2005-344603 A).